Brewer's yeast (also known as
brewers yeast or brewing yeast) can mean any live yeast
used in brewing. It can also mean yeast obtained as a
by-product of brewing, dried and killed, and used as
nutritional yeast.

Beer brewers classify yeasts as
top-fermenting and bottom-fermenting. This distinction
was introduced by the Dane Emil Christian
Hansen.

Top-fermenting yeasts (so-called because they
float to the top of the beer) can produce higher alcohol
concentrations and prefer higher temperatures. An
example is Saccharomyces cerevisiae, known to brewers as
ale yeast. They produce fruitier, sweeter, ale-type
beers. Bottom-fermenting yeasts ferment more sugars,
leaving a crisper taste, and work well at low
temperatures. An example is Saccharomyces uvarum,
formerly known as Saccharomyces carlsbergensis. They are
used in producing lager-type beers. Brewers of wheat
beers often use varieties of Torulaspora
delbrueckii.

These fall into two main
categories:

Saccharomyces uvarum or S.
carlbergensis

Saccharomyces cerevisiae

To ensure
purity of strain, a 'clean' sample of the yeast is
stored in a laboratory environment at refrigerated
temperature. After a certain number of fermentation
cycles, a full scale propagation is produced from this
laboratory sample. Typically, it is grown up in about
three or four stages using sterile Brewing wort and
oxygen.

Nutritional yeast
"Brewer's yeast"
can also refer loosely to any nutritional yeast. S.
cerevisiae is the usual species for this
purpose.